(1) Field of the Invention
This invention relates to a novel process for the formation of a fluoroplastic coating on the surface of zinc, a zinc alloy or a zinc plating to impart high corrosion resistance, chemical resistance and heat resistance to the substrate.
(2) Description of the Related Art
Formation of a coating of a fluoroplastic on zinc, a zinc alloy or a zinc plating, which may hereinafter be called a "zinc substrate", has heretofore been considered unsuitable because the melting point of zinc is low in spite of the need for a rather high baking temperature for the formation of the coating from the fluoroplastic and zinc reacts with the fluoroplastic to form a compound which impairs the adhesion between the zinc substrate and the coating.
With a view toward overcoming the above difficulties, Japanese Patent Publication No. 43045/1976 discloses a process for the formation of a fluoroplastic coating having practically sufficient hardness, corrosion resistance and chemical resistance. According to the process, a zinc alloy is subjected to chromate treatment, applying a coating formulation of a special fluoroplastic of the non-sticking two-component type (for example, "TEFLON S", trade name; product of E.I. du Pont de Nemours & Co., Inc.) on the resultant film of the chromic acid and then baking the fluoroplastic at a relatively low temperature not impairing the physical properties of the zinc alloy, namely, at 140.degree.-170.degree. C.
Japanese Patent Publication No. 60434/1982 discloses a process for coating a zinc-plated metal article with a protective layer of a fluoroplastic. According to the process, the zinc-plated article is treated in a chromate solution having a pH of 2 or lower and containing formic acid as a reducing agent to form a chromate layer, coating the chromate-coated article with a dispersion of polyvinyl fluoride in a high boiling-point solvent and then subjecting the resultant article to a heat treatment at a temperature of at least 100.degree. C., whereby the zinc-plated metal article is coated with the protective layer of the fluoroplastic.
On the other hand, Japanese Patent Publication No. 12250/1968 discloses the formation of a corrosion and abrasion-resistant coating on a surface of zinc or a zinc alloy by forming an anodic oxide film without relying upon a coating layer of a fluoroplastic.
In Japanese Patent Publication No. 12250/1968, the anodic oxidation of zinc or the zinc alloy is conducted using an electrolyle which contains phosphate anions and chromate anions as primary ions along with anions of at least one kind selected from molybdate anions, tungstate anions and vanadate anions and cations of at least one kind selected from sodium cations and potassium cations. The principal feature of the process resides in the use of low-voltage power for alternating current electrolysis.
Of the above-described conventional processes for the formation of a fluoroplastic coating on zinc, a zinc alloy or a zinc plating, it is essential for the former process (Japanese Patent Publication No. 43045/1976) to use a coating formulation of a modified fluoroplastic such as TEFLON S (trade name; product of E.I. du Pont de Nemours & Co., Inc.). Difficulties are therefore encountered upon selection of a resin to be employed in the process. Limitations are imposed in this regard. The above modified fluoroplastic contains one or more other resin components and is hence inferior in heat resistance and corrosion resistance. On the other hand, the latter process (Japanese Patent Publication No. 60434/1982) requires that the dipping in the dispersion of the fluoroplastic must be conducted while the chromate film has not been fully dried subsequent to the chromate treatment, in other words, while water still remains in porous hollow interstices of the chromate film layer. This is to facilitate the replacement of the remaining water with the dispersion of the polyvinyl fluoride in the high boiling-point solvent in the next step. Work-related limitations are thus imposed, leading to the need for a continuous production system which can continuously and successively perform all steps ranging from electroplating to coating.
In addition, the process of Japanese Patent Publication Nos. 43045/1976 and 60434/1982 referred to above are both adapted to apply a coating of a fluoroplastic on a chromate film which has been formed on a zinc substrate. The chromate film is formed in the form of a gel on the surface of the zinc substrate. The fluoroplastic coating cannot exhibit its inherent heat resistance fully at high temperatures and limitations are hence imposed on the conditions for the use of the product, although the fluoroplastic coating can exhibit superb effects on the corrosion resistance at room temperature.
On the other hand, Japanese Patent Publication plastic. Mere formation of an anodic oxide film can by no means bring about the above-described various properties of fluoroplastics.